water Article Dependence of Long-Term Dynamics of Zooplankton in the Ob River on Interannual Changes in Hydrological and Hydrochemical Parameters Nadezhda Yermolaeva * , Serafima Dvurechenskaya, Vladimir Kirillov and Aleksandr Puzanov Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences, Molodezhnaya Str. 1, 656038 Barnaul, Russia; serafi[email protected] (S.D.); [email protected] (V.K.); [email protected] (A.P.) * Correspondence: [email protected]; Tel.: +7-952-903-11-59 Abstract: Here we summarize a long-term study on qualitative and quantitative composition of zooplankton (Cladocera, Copepoda, Rotifera) in the Ob River. We carried out these investigations at 13 sampling stations of the Middle and Low Ob in the years 1994, 1996, 1999, 2001, 2002 and 2009. It was found that the species richness of all zooplankton and abundance of cladocerans and rotifers is significantly determined by the temperature conditions of the month preceding sampling. In contrast, among other factors, we revealed that pH decreases as well as phosphate and nitrate concentrations increase zooplankton abundance. Dissolved oxygen and oxidizable organic substances (BOD ) were 5 positively correlated to copepod population levels (according to abundance and biomass); on the other hand, an increase in difficult-to-oxidize substances (COD) inhibited their development. During Citation: Yermolaeva, N.; Dvurechenskaya, S.; Kirillov, V.; this study, we found that high water levels had a positive influence on zooplankton richness in river Puzanov, A. Dependence of itself probably due to being downstream from Ob River floodplain lakes. Long-Term Dynamics of Zooplankton in the Ob River on Interannual Keywords: zooplankton; long-term dynamics; water level; Ob River; environmental factors Changes in Hydrological and Hydrochemical Parameters. Water 2021, 13, 1910. https://doi.org/ 10.3390/w13141910 1. Introduction Solving the problem of influence of hydrological and hydrochemical characteristics of Academic Editors: Xin’an Yin, streams as well as climate change on qualitative and quantitative features of hydrobionts is Xufeng Mao, Jianguo Zhou and of particular importance for working out a strategy for water use and protection of water Zhengjian Yang resources on the worldwide scale, including little studied large waterway such as the Ob River. In contrast to lentic freshwater systems, understanding the influence of hydrological Received: 7 June 2021 and chemical characteristics of streams on zooplankton is difficult as the environmental Accepted: 6 July 2021 Published: 9 July 2021 conditions of the flow change rapidly. The Ob is the largest watercourse in Western Siberia and one of the largest rivers in Publisher’s Note: MDPI stays neutral the world. The source of the river is the confluence of the rivers Biya and Katun; the river with regard to jurisdictional claims in mouth is the Ob Bay in the Kara Sea of the Arctic Ocean. The river is 3650 km long; its 2 published maps and institutional affil- catchment area makes up 2,990,000 km . The Ob basin (about 85%) is mainly located on the iations. West Siberian Plain, crossing all natural zones of temperate latitudes (steppe, forest-steppe and taiga). In its low reaches, the Ob River flows in the permafrost zone. Dams do not tangibly regulate the flow of the river. In the upper reaches, only the Novosibirsk reservoir (near the city of Novosibirsk) with its small regulating capacity (4.4 km3) is functioning [1]. The Ob River is characterized by considerable interannual fluctuations in water levels [2], Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. which depend on its annual hydrological regime. Changes in flow volumes and water This article is an open access article levels make an effect on physical properties of the flow and the chemical composition of distributed under the terms and waters [3] thus affecting zooplankton as well [4–8]. conditions of the Creative Commons Unlike fish and other large river organisms, zooplankton, including Cladocera, Cope- Attribution (CC BY) license (https:// poda and Rotifera, comprise a diverse group of heterotrophic organisms transported creativecommons.org/licenses/by/ passively by flowing waters [9–14]. Filter-feeding organisms are the main part of the zoo- 4.0/). plankton community of rivers [7,8,15–18]. They feed on phytoplankton, bacterioplankton, Water 2021, 13, 1910. https://doi.org/10.3390/w13141910 https://www.mdpi.com/journal/water Water 2021, 13, 1910 2 of 23 detritus and each other; thus, they are critical elements in both the classical and microbial loop food web. Their nutrients and energy are transfer to higher trophic levels, including fish and insects. They also contribute to purification of water bodies by removing sus- pended dead organic matter. Zooplankton is a chain in matter and energy transformation and an important factor of water quality formation [16,17,19–22]. Numerous researchers report that physical-chemical factors, such as concentrations of biogenic elements (i.e., phosphorus, nitrogen), organic substances (according to BOD5), dissolved oxygen, pH, water temperature and a number of other parameters (flow rate, discharges, water levels, etc.), are closely related with the development of different zooplankton species [23,24]. The composition of the zooplankton community has been used to assess water quality and is often used as a pollution indicator [25–29]. Therefore, studies of the long-term dynamics of zooplankton in large river systems are promising because they allow scientists to assess changes occurring in the aquatic ecosystem. In fact, there are only few studies of zooplankton in large rivers and their dependence on environmental parameters. It happens due to the complexity of studying large river systems, including their huge extent. For studying seasonal dynamics, not lengthy sites of river are usually used [7,18,30–32]. Large-scale studies are conducted in spring-summer, i.e., a period that characterized by high biological activity, relatively stable flows and mild weather favorable for sampling to investigate spatial heterogeneity [6,8,11,12,24,33–38]. Though a large number of researchers have shown a scientific interest in the Ob River for many decades, just few separate hydrological and hydrochemical [39–48] as well as hydrobiological [15,49,50] issues have been studied. In particular to date, the species composition of zooplankton has been studied in sufficient detail only in some parts of the Ob [15]. However, complex studies that would allow assessing the impact of various factors on the qualitative and quantitative characteristics of zooplankton in the Ob River were not conducted. This project is focused on studying the long-term dynamics of zooplankton of the Ob River related to interannual changes in hydrological and hydrochemical parameters. 2. Materials and Methods Investigations were carried out in the years 1994, 1996, 1999, 2001, 2002 and 2009 during route surveys of the Middle and Low Ob River section, namely, from the dam of the Novosibirsk HPP to the village of Karymkary (Figure1). The studied section of the river is almost entirely located in the taiga landscape zone [2]. Here, the river receives large tributaries (the Tom, Chulym, Ket’, Tym, Vakh, Vasyugan, Irtysh, etc.), which have an essential impact on its hydrochemical and hydrobiological parameters both in the sites of confluence and downstream. Taking into account the time, required for carrying out the work in all sites, the speed of the research vessel along the route was commensurable with the rate of water mass. It was assumed that we studied the same water mass undergoing transformation when it moves downstream and is influenced by the lateral inflow, changes in the catchment features, etc. Sampling was made in the last decade of August during the period of water stage stabilization and maximum development of crustacean zooplankton. Hydrobiological and hydrochemical sampling was made concurrently at the same sites (Table1, Figure1). Water 2021, 13, 1910 3 of 23 Figure 1. Schematic map of sampling sites (sites numbers correspond to those specified in Table1). Water 2021, 13, 1910 4 of 23 Table 1. Location of sampling sites and hydrometeorological stations. Km No Sampling Site Names Sampling Site Coordinates From the OB Source Hydrochemical and hydrobiological samples 0 Novosibirsk HPP 54◦5005100 N 82◦5903000 E 678 1 Dubrovino 55◦2804200 N 83◦1603600 E 785 2 7 km above Tom River mouth 56◦4405600 N 84◦2404700 E 963 3 Tom River mouth 56◦5302100 N 84◦2704200 E 987 4 10 km below Tom River mouth 56◦5801400 N 84◦2400800 E 997 5 1 km above R.Chulym mouth 57◦4205800 N 83◦5101500 E 1132 6 R.Chulym mouth 57◦4303000 N 83◦4903500 E 1133 7 3 km below R.Chulym mouth 57◦4204000 N 83◦4602800 E 1136 8 1 km above Nizhnevartovsk 60◦5003700 N 76◦3802800 E 1940 9 1 km below Nizhnevartovsk 60◦5103300 N 76◦2305500 E 1956 10 30 km above R.Irtysh mouth 60◦1004600 N 69◦1101700 E 2430 11 R.Irtysh mouth 61 0404800 N 68 4905000 E 2500 Hydrometeorological stations Dubrovino 55◦2804200 N 83◦1603600 E 785 Kolpashevo 58◦1800400 N 82◦5401900 E 1250 Aleksandovskoye 60◦2603600 N 77◦5205700 E 1840 Field measurements included Secca disc visibility (SDV), temperature, electrical con- ductivity and pH using an ANION 7051. Dissolved oxygen concentration was detected by the Winkler method. We also collected water samples from the upper 50 cm layer using a 5-L bathometer for laboratory analysis of total phosphorus (TP), ammonium, nitrates and chemical oxygen demand (COD). The samples were filtered under argon pressure through White Tape paper filters with a pore size of 5–8 µm to remove coarse suspension and then frozen at −18◦C before the delivery to the laboratory (for 14 to 30 days). Subsequently, after thawing, the water parameters were identified in the laboratory according to standard methods: ammonium, nitrates and phosphate—by photometric method; COD—by the method that based on oxidation of organic substances with an excess of potassium dichro- mate in a sulfuric acid solution when heated at the presence of a catalyst (silver sulfate).